A NEW 12-KILODALTON DIMER ASSOCIATED WITH PRE-TCR COMPLEX AND CLONOTYPE-INDEPENDENT CD3 COMPLEX ON IMMATURE THYMOCYTES

We have characterized the pre-TCR/CD3 complex and the clonotype-indepe ndent CD3 (CIC) complex expressed on the cell surface of the immature cell line KKF which we previously identified as the cell line expressi ng only TCR beta-chain associated with the CD3 complex on the cell sur face. We now show that KKF expresses the pT alpha-beta heterodimer-CD3 complex and also the CIC complex on the cell surface by surface bioti nylation. In addition, not only the CIC complex but also the pT alpha- beta complex was found to be associated with the molecular chaperon ca lnexin. Whereas the CD3 zeta-chain was very weakly associated with the pre-TCR/CD3 complex, a 12-kDa molecule (designated pTAC12 as pre-TCR- associated chain) was more stably associated as a dimer with the pre-T CR complex as well as the CIC complex on KKF cells. The significance o f pTAC12 dimer in the CD3 complex of immature thymocytes in vivo was d emonstrated by coprecipitation of pTAC12 with the CD3 complex in thymo cytes from SCID mice. These results show that pTAC12 is a component of the pT alpha-beta complex as well as the CIC complex, and they sugges t that pTAC12 may play a role in signaling through and/or in the expre ssion of the pre-TCR/CD3 complex.